We had this problem a year ago - no water coming out of the tap, yet we could hear the pump working and the water sloshing around in the tank. Turned out that the pump had become dislodged from its seating inside the tank, and because the pump was moving around in the water when being used, the exit pipe from the pump had eventually slipped off the exit connector inside the tank, so the water was just circulating around inside the tank.
Access the tank through the "wardrobe", lift the circular cover, unscrew the inspection lid and disconnect the power lead. It's a good idea to open the boot as this will trigger the boot light which is next to the tank and illuminate it for you. The bottom of the pump should be sited in a shallow cup on the base of the tank, to the right of centre looking down into the tank. You need to feel for this as it is not obvious when looking in. A pipe from the pump should be pushed onto the exit connector in the top of the tank to the right of the inspection access. Initially we had problems of the hose coming off again until we discovered that the pump needed to sit in the cup. Additionally we also decided to try to make a better job of keeping the hose on the exit connector. A metal jubilee clip that might rust was not a good idea and we ended up using a plastic gardening tie that could be tightened one-handedly. A year on and no further problems. We do take care filling the tank - pushing in a hosepipe probably caused the pump to become dislodged in the first place.